Non-resonant stub supports for slow wave circuits



March 18, 1969 R. J. ESPINOSA NONRESONANT STUB SUPPORTS FOR SLOW WAVECIRCUITS GEZEDSE iv v cow 0 com- MM o a A I v ow m mm M12227 m Y w w a Ym r u\ 0 -o 2 dmwo m P... 0 oo JEN J T n T m 0 GE 2650 m A .021 48.5282311% woo: Q 4 GEBZMDSE m8? mEwm R /k 8 o 8 w m? ,m m P LE L 0?" m y m- J-n v a J o V GE w m taoma mEwEoIw m 0E N GE n m 6 m8: mam N F 21 AT .m gi r 0 United States Patent ABSTRACT OF THE DISCLOSURE Troublesomespurious oscillations caused by prior art M4 stub supports can beeliminated in ring-andbar type slow wave circuits by dimensioning thestub supports to have a length L defined by where A is the wavelength atthe center frequency of the operating band.

This application is a streamlined continuation of application Ser. No.384,668, filed July 23, 1964, now abandoned.

This invention relates in general to high frequency electron dischargedevices and more particularly to such devices incorporatinghelix-derived slow wave circuits of the ring-and-bar type. These highfrequency electron discharge devices are particularly useful infrequency generators, broadband high power amplifiers, etc. Ahelixderived slow wave circuit of the ring-and-bar type such as shown inUS. Patent 2,937,311, by M. Chodorow is an excellent vehicle forpropagating fairly high powers of traveling wave electromagnetic energyfor interaction with electron beams and subsequent amplificationthereof. The thermal capacity of this particular type of circuit such asshown, for example, in US. Patent 2,853,642 by C. K. Birdsall et al. isenhanced to a great extent by the inclusion of a stub support for theslow wave circuit as shown in Birdsall et al. conventionally, the lengthof the support stubs is made one-quarter wavelength as determined at thecenter of the operating passband of the circuit. However, as higher andhigher electron beam powers are used, stability of tubes utilizing suchcircuits has become a pronounced factor in power handling capabilitiesof such tubes.

Higher order circuit modes become increasingly ditficult to stabilizeand may seriously affect the performance as the power increases. Forexample, voltage build up on the slow wave circuit due to high electricfields caused by the buildup of resonant modes and consequent energyloss to these modes limits available energy for the fundamental modethereby providing a limitation on useful tube operating power level orresulting in destruction of the tube. A slow wave circuit and associatedtraveling wave tube may be limited seriously unless stabilizationtechniques are utilized as multi-kilowatt power levels are reached.

An analysis of a few higher order circuit modes such as the ring modeand stub mode and techniques suitable for eliminating or at leastreducing the effects of such resonant circuits modes are given in US.patent application Ser. No. 306,570 by Robert J. Espinosa and John A.Ruetz now US. Patent No. 3,335,314 and assigned to the same assignee asthe present invention. In that particular application the stub mode wasstabilized through the utilization of a tapered stub length along theaxial extent.

of the tube. This resulted in increasing the 1,, (start oscillationcurrent) for the stub mode and resulted in greatly increasedstabilization of a pulse amplifier utilizing such a tapered technique.The stub mode as can be defined as a higher order resonant circuit modewherein the length of the support stub determines the upper cutofffrequency of the mode. By way of definition the fundamental or lowestorder mode of propagation for any periodic slow wave circuit ischaracterized by a particular field pattern in a plane transverse to thedirection of propagation, which field pattern is independent of positionalong the axis of propagation. Higher order modes of propagation, suchas the stub mode, are herein defined as propagation modes other than thefundamental mode which are also characterized by a particular fieldpattern in a plane transverse to the direction of propagation and whichfield pattern is individually distinct and different from other modes.

The present invention provides another solution to the problem ofstabilizing a high power traveling wave tube incorporating a helixderived stub-supported ring-and-bar slow wave circuit. Through adetailed measurement of the high frequency passbands of stub-supportedring-andbar circuits and through analysis of the tube operating data, ithas been determined that the stub length is found to be a determinativefactor in the stabilizaion of a stubsupported ring-and-bar slow wavecircuit. The tapered stub technique described in the aforementionedapplication Ser. No. 306,570 has been found to be quite adequate tostabilize a high power traveling wave tube incorporating astub-supported slow wave circuit. However, when such a tube is driven tosaturation over its operating band a reduction in power output isobserved at the middle of its operating band. This indicates that thetapered technique disclosed previously is not exactly optimum as far ascomplete stabilization of the stub mode is concerned.

The present invention provides a solution to such depressed saturatedpower output problems encountered with the tapered stub-supportpreviously mentioned by a deviation from the conventional /4 stublengths used in all prior art stub-supported ring-and-bar circuits. Thisdeviation which can take place either above or below the conventionalquarter wavelength stub length, although preferably below for band widthand cost considerations, as well as enhanced stability properties willbe described in more detail hereinafter. The present invention hasdetermined that at approximately /6 stub lengths as determined at f(center frequency of. the operating band) the stub mode can be moved upin frequency sulficiently to prevent second harmonic interaction of thesignal fre quency and beam which might otherwise induce stub modeoscillations. The present invention also teaches a preferred range ofstub lengths, such as the following: h to A, and A to A which willencompass the desired useful stu'b parameters for stabilization.

The present invention further teaches, in regard to stub supports havinginductive and/or capacitive perturbations therebetween that deviationsfrom a /4 as expressed in terms of free space wavelength may be ratherpronounced and a plane electromagnetic wave traveling in the perturbedregion at some velocity other than the velocity of the light still fallswithin our teachings and distinguishes over the prior art quarterwavelength teachings while providing stub modesuppression when expressedas follows:

Said stubs having length dimensions falling within the following limits:

)\/l6 L i and y h L )\/3 where L is the stub length and is the physicallength of the path taken along the surface of the stub as measured fromthe end of the stub at the outside diameter of the ring circuit to thestub base or root and c=c/f where f is center frequency of the passbandof the stub supported circuit and c is the velocity of a planeelectromagnetic 3 wave in the dielectric medium or mediums between saidstubs averaged over the length of the stub L.

It is therefore, a principal object of the present invention to providea novel slow wave circuit and improved high frequency electron dischargedevices.

A feature of the present invention is the provision of a helix-derivedslow wave circuit having novel supporting techniques therefor.

Another feature of the present invention is the provision of ahelix-derived ring-and-bar type slow wave circuit having a stub supportof approximately 6 as measured at the center of the operating frequencyband of the circuit.

Another feature of the present invention is the provision of astub-supported, helix-derived slow wave circuit of the ring-and-bar typewherein the stub lengths fall within the where L is the stub length andt is determined at 1 (center frequency of the operating band of thecircuit).

Another feature of the present invention is the provision of astub-mounted helix-derived slow wave circuit of the ring-and-bar typewherein the stubs have length dimensions falling within the followinglimits:

where L is the stub length and is the physical length of the path takenalong the surface of the stub as measured from the end of the stub atthe outside diameter of the ring circuit to the stub base or root and\=c/f where f is center frequency of the passband of the stub supportedcircuit and c is the velocity of a plane electromagnetic wave in thedielectric medium or mediums between said stubs averaged over the lengthof the stub L.

Another feature of the present invention is the incorporation of any ofthe slow wave circuits mentioned in the aforementioned features in ahigh frequency electron discharge device utilizing travelling waveelectromagnetic interaction therein.

These and other features and advantages of the present invention willbecome more apparent after a perusal of the following specificationtaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a longitudinal cross-sectional view, partly in elevation, of atravelling wave tube incorporating a novel slow wave circuit of thepresent invention;

FIG. 2 isa cross sectional view taken along the lines 22 of FIG. 1;

FIG. 3 is an illustrative w-fl diagram of the slow wave I circuitsdepicted in FIGS. 1 and 2;

FIG. 4 is an illustrative graphical portrayal of peak power output vs.frequency of a slow wave circuit such as depicted in FIGS. 1 and- 2 withx.,/ 4 stub lengths; and

FIG. 5 is an illustrative graphical portrayal of peak power output vs.frequency of a slow wave circuit incorporated in a traveling wave tubesuch as depicted in FIGS. 1 and 2 wherein stabilization techniques ofthe present invention are utilized.

Referring now to FIG. 1 of the drawing, there is shown a traveling wavetube 7 having an electron gun structure 8, conductive body 9, collector10, anode portion 11, helix-derived ring-and-bar type slow wave circuit12 defining a series of interaction gaps 13 between spaced rings 14,supporting stubs 15 and RF. input and output coupling means 16 and 17,respectively. The traveling wave tube depicted in FIG. 1 is axiallyaligned along a central axis of propagation 7. An electron beam 18emanating from the electron gun portion 8 as shown by dotted linestravels along the central axis to the collector 10. Since theinteraction between an electron beam and RF. energy on slow wavecircuits is well known, no further explanation thereof for purposes ofamplification is required. A conventional circuit sever 19 is disposedbetween slow wave circuit sections and the circuits are terminated by apair of dissipative loads 20, 21 in order to absorb unwanted R.F. energyin a manner well known in the art. Preferably the conductive body 9 isprovided with a pair of cooling channels 22, 23 as best seen in FIG. 2for the purposes of increasing thermal dissipation and thus increasingpower handling capabilities of the circuit. Conventional loading ridges24, 25 such as described in US. patent application Ser. No. 295,605 byJohn W. Sullivan filed July 15, 1963, now US. Patent No. 3,142,777, areadvantageously incorporated for purposes of broadbanding as described inmore detail in the aforementioned Sullivan application. The supportstubs 15 are preferably tapered per the techniques taught in theaforementioned Sullivan patent application.

In FIG. 3 an illustrative w-B diagram of the pass bands of the circuitdepicted in FIG. 1 is given. V=c is a characteristic of the velocity oflight while V=u is the operating beam voltage or beam velocitycharacteristic. The characteristic labeled fundamental mode is theoperating mode of the tube utilizing a stub-supported ringand-barcircuit as shown in FIGS. 1 and 2. Dotted characteristic A shows theresultant fundamental mode characteristic when a short stub ofapproximately A /G is utilized. Characteristic B shows the depression ofthe fundamental mode when a large or longer length stub support isutilized, as for example, x Dotted characteristic A shows the resultantstub mode characteristic when a short stub of approximately /6 isutilized, showing that the stub mode is not intercepted by u Dottedcharacteristic B shows the resultant stub mode characteristic when astub length of approximately 71 A is used, showing that the stub modeoccurs far above 21r but is still intercepted by a therefore, there isstill a possibility of stub mode interaction if some other means ofoscillation suppression is not used. The characteristic labeled stubmode depicts a mode which, as evidenced by the characteristic, is verydispersive and has a very low group velocity, thus showing the resonantcircuit characteristics thereof. It is evident that a M 4 stub length asdetermined at i will cause stub mode oscillations at second harmonicfrequencies as shown by the intersection of u and the stub modecharacteristic (see diagram in FIG. 3).

The present invention, when utilizing a /6 stub length, results inmoving the stub mode up in frequency high enough to prevent synchronousinteraction with the operating beam velocity as evidenced by thecharacteristic labeled stub mode of the short stub circuit. Examinationof fundamental mode characteristics A and B shows the perturbationthereof or deviation from the quarter wavelength stub length fundamentalmode. The fundamental characteristic for a /6 stub, since it is slightlyless dispersive relative to the quarter wavelength characteristic,provides advantageous results with regard to operating tube bandwidth aswell as increased stabilization of a travelling wave sectionincorporating the short stub technique as depicted herein.

The slow wave circuit depicted in FIGS. 1 and 2 when utilizing thefollowing parameters, stub length equal x 6, ring exterior radius equalto .075 stub apex angle equal to 22 /2 waveguide height of approximatelyMI 4, distance between loading ridge and exterior ring radius of 025%,when utilized in conjunction with a tube which had a saturated gain ofdb across a 16% frequency bandwidth at a beam velocity of approximately36 db and 14 amps beam current resulted in the exemplary saturated R.F.output characteristics depicted in FIGS. 4 and 5 with and without,respectively, the stub length control techniques in regard tostabilization as taught by the present invention. Examination of FIG. 4which depicts exemplary saturated output characteristics for a A /4 stublength even with a tapered stub stabilization technique as described inUs. Ser. No. 306,570 shows a severe drop in power output at f,,. When ax /6 stub length is employed it is quite evident that the depressedpower output at i is eliminated. Quite obviously, upon examination ofFIG. 5, no reduction in bandwidth or where L is the stub length in aplane transverse to the Z axis and A is determined at f (centerfrequency of the band of frequencies to be propagated along thecircuit).

It is to be noted that the short stub is preferred as it easilyeliminates any possibility of correspondence between signal energy withthe operating band and frequencies at which the stub mode propagatesincluding second harmonics of any frequency within the operating band.Furthermore, n does not intercept the short stub characteristic thusassuring freedom from noise excitation of the stub mode.

The long stub length 7 easily prevents any correspondence between signalenergy at frequencies within the operating band as well as secondharmonics of any frequency within the operating band and frequencies atwhich the stub mode propagates 2) but does not assure completestabilization since noise excitation by the beam is still possible asevidenced by the crossover between the M and the stub mode of long stubcircuit (B) characteristics.

However, just the elimination of stub mode oscillations by secondharmonics of frequencies within the operating band provides a tremendousboost for the tube designer when considered of and by itself.

As mentioned previously, in regard to stub supports having inductiveand/or capacitive perturbations therebetween such that deviation from a/4 as expressed in terms of free space wavelength may be ratherpronounced, a plane electromagnetic wave traveling in the perturbedregion at some velocity other than the velocity of light still fallswithin our teachings and distinguishes over the prior art quarterwavelength teachings while providing stub mode suppression whenexpressed as follows:

Said stubs having length dimensions falling within the following limits:

where L is the stub length and is the physical length of the path takenalong the surface of the stub as measured from the end of the stub atthe outside diameter of the ring circuit to the stub base or root and\=c/f where i is center frequency of the passband of the stub supportedcircuit and c is the velocity of a plane electromagnetic wave in thedielectric medium or mediums between said stubs averaged over the lengthof the stub L.

Quite obviously the stub supporting stabilization techniques describedherein are improvements over the stub mode stabilization techniquesdisclosed in aforementioned U.S. Ser. No. 306,570.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a high frequency electron discharge device for propagatingelectromagnetic energy within a band of frequencies, said deviceincluding a helix-derived slow wave circuit of the ring-and-bar type,the improvement comprising a plurality of conductive stub supportmembers attached to said slow wave circuit and spaced along the axialextent thereof, said stubs having length dimensions L which fall withinthe following limits:

where L is the stub physical length, =free space wavelength at i (centerfrequency of the operating band of frequencies of said device).

2. In a high frequency electron discharge device for propagatingelectromagnetic energy within a band for frequencies, said deviceincluding a helix-derived slow wave circuit of the ring-and-bar type,said slow wave circuit having a plurality of conductive stub supportmembers attached thereto and spaced along the axial extent thereof,wherein said stubs have length dimensions which are approximately A /6,where A is free space wavelength at f (center frequency of the operatingband of frequencies of said device).

3. In a high frequency electron discharge device for propagatingelectromagnetic energy within a 'band of frequencies, said deviceincluding a slow wave circuit comprising a plurality of spacedconductive rings interconnected by conductive stubs, the improvementcomprising said stubs having a length as determined at f (centerfrequency of said band of frequencies) such that there is nocorrespondence between the frequencies within the operating band and anysecond harmonics of any frequency Within the operating band and thefrequency at which the stubs are A2 A where A is free space wavelength.

4. A high frequency electron discharge device for propagatingelectromagnetic energy within a band of frequencies, said deviceincluding a vacuum envelope, gun means for generating a beam ofelectrons at an upstream end of said envelope, collector means forcollecting said electrons at a downstream end of said envelope, anddisposed along the path of said beam, a helix-derived slow wave circuit,said slow wave circuit having a plurality of conductive stub supportmembers attached thereto and spaced along the axial extent thereof, saidstubs having length dimensions L falling within the following limits:

where L is the stub physical length as measured between the outsidediameter of the slow wave circuit and the base or root and where f(center frequency of said band of frequencies) and c=average velocity ofa plane electromagnetic wave in the mediums between said stubs averagedover the length of the stubs.

